Excessive hair loss (alopecia) is defined as losing enough hair that a person has visibly thin or balding patches. A number of events or circumstances can interrupt the normal cycle of hair growth and cause hair loss to occur (e.g., heredity, hormones, medication). Hair loss may be temporary or permanent, depending on the cause. Excessive hair loss caused by heredity is by far the most common cause in both men and women, so much so that it is often considered normal instead of a medical condition.

Hair loss is generally classified as either scarring or nonscarring. Nonscarring refers to hair loss such as male pattern hair loss, where the scalp is not damaged but hair is lost. Scarring hair loss occurs at scarred or damaged areas of the scalp and is much less common than nonscarring hair loss. It is usually caused by trauma (e.g., burns, physical injury) but may be caused by disease as well (e.g., skin cancer, lupus).

Hair loss is usually noticeable by simply looking into a mirror. An unusually high number of hairs discovered on a pillow or hairbrush are also common indicators that abnormal hair loss may be occurring. There is no cure for hereditary hair loss. However, treatments are available that may grow hair or slow the progression of hair loss, with medications and surgery being the most widely used (e.g., minoxidil, hair transplantation).  

Often, the ability to prevent or stop hair loss largely depends on the underlying cause. Loss of hair due to a stressful event or disease (telogen effluvium) does not usually persist once the underlying cause has ended or been removed. Non-hereditary hair loss is far less common but often much more easily preventable. It may be possible to prevent further hereditary hair loss or to regrow hair in some patients if it is treated early, though current treatments do not work for everyone.